Abstract

Doublet structure in the 5.76 eV absorption band of thallium activated KDP single crystals has been investigated at various temperatures between 4.2 K and 346 K. Its temperature dependence indicates that the structure is closely related to the ferroelectric properties of KDP. Energy separation of the doublet components is proportional to \sqrtT above the transition point Tc. This signifies that the structure is caused by the dynamic Jahn-Teller effect in the degenerate excited state. The structure below Tc, however, is due to the static crystal field, symmetry of which is reduced by the ferroelectric transition. Peculiar variation of the separation in a narrow temperature range immediately below Tc seems to reflect lattice deformation through the phase transition.